1. Field of the Invention
The present invention relates to methods and systems for making global observations of the Earth at sub-kilometer spatial resolutions in real-time, where real-time refers to a delay of not more than two minutes total for creating, refreshing and distributing each image. More particularly, the present invention is directed towards methods, apparatuses and systems that provide real-time coverage of at least 70% of the observable Earth surface at a spatial resolution of less than 1 kilometer.
2. Discussion of the Background
Over the last 30 years, since the first weather monitoring satellite was placed in geostationary earth orbit (GEO), various satellite systems have been used to monitor features of the Earth. The reason is that at GEO the relative motion of the Earth and the satellite is nulled, provided that the GEO orbit is in the Earth's equatorial plane. Accordingly, consistent images may be taken of the portion of the Earth's surface and atmosphere that fall within the footprint of the satellite.
In the Western hemisphere, weather forecasting methods rely heavily on data supplied by the Geostationary Operational Environmental Satellites (GOES) series, operated by the National Oceanic and Atmospheric Administration (NOAA). The GOES series was developed from the prototype "Advanced Technology Systems" 1 and 3 (ATS-1, -3) launched in 1966 and 1967, respectively. These and all subsequent systems have been implemented with scanning imaging systems that are able to produce full disk images of the Earth at 1 km resolution in about 20-30 minutes.
The newest of the GOES satellites (8, 9 and 10) are 3-axis stabilized and are configured to observe the Earth with 1 panchromatic visible and 4 infrared imaging systems per satellite. The visible imaging systems use a "flying spot" scanning technique when a mirror moving in two axes, East-West and North-South, scans a small vertically oriented element of the fully viewable scene (the instrument's full area of regard) across an array of eight vertically arranged silicon pixels. The individual pixel field of view is about 30 .mu.rad. Each scene element is sampled for just under 50 microseconds. In order to support this slow scanning method, the GOES satellite payload stability must be extraordinarily high so that almost no relative movement occurs between any one scan line of the samples. Accordingly, the payload pointing does not nominally deviate further than 1/3 of a pixel during an entire, 1 second duration scan. Because there are over 1,300 scan lines to create a full disk image it takes about 22 minutes to create the full image. Operationally, a full disk sampling technique is actually done once every three hours, to allow more frequent coverage of an entire visible hemisphere rather than a more frequent sampling of smaller regions.
During normal operation, GOES series satellites provide gray-scale and infrared images of different portions of the Earth at between 5, 15 and 30 minute intervals. Limited regions may be sampled as frequently as about once per minute, during "super rapid scan operations" (SRSO). In practice, SRSO operations are rarely used because coverage of other areas is too important to be neglected for long periods of time. Moreover, significant Earth-based events that occur during lapses in coverage of a particular region may be missed. In other words, the satellites sensor may be looking an uneventful portion of the Earth's surface when the significant activity is occurring at another portion of the Earth's surface. Furthermore, as recognized by the present inventor, phenomena that may occur at night can only be seen in the infrared channels, which have a much coarser spatial resolution than the visible channel and otherwise are subject to the same limitations that are inherent in a scanning system.
GOES satellites provide a system that is optimized for monitoring cloud motion, but is far less suitable for observing other GEO physical events. At visible wavelengths, clouds are efficient diffuse mirrors of solar radiation and therefore appear white with variations of brightness seen as shades of gray. Color, enhancing the contrast and visibility of the Earth's surface background, may actually detract from cloud visibility in a scene. Moreover, adding color may triple the amount of information and thus size of a digitized image, which creates a burden on the transmission demands for the broadcast portion of the satellite system. Furthermore, observations of significant, but perhaps transient phenomena that occur in time scales of seconds or minutes (such as volcanoes, lightening strikes or meteors) may be late or not observed at all. Accordingly, the information provided from systems like the GOES system is somewhat unreliable because it is not able to provide a high-resolution "watchdog" service that reliably reports real-time information over a significant portion of the Earth's surface. Also, "video" style loops created from successive images having relatively coarse temporal resolution may lack the continuity needed to provide truly reliable information if cloud movements between image samples are much greater than a pixel dimension. The temporal coherence among the pixels of a scanned image and between the co-registered pixels of successive images will degrade as the time required to create the image and the elapsed time interval between scans increases. These effects have a significant adverse impact on the fidelity of any "image" created to represent the state of the Earth at a given moment, but particularly harmful to attempts to build animations using successive co-registered scanned images of a given area.
Referring to FIG. 1, coverage area are shown for various weather satellites in addition to the GOES satellites. The GMS-5, parked at 140.degree. East longitude, is a Japanese weather satellite showing a coverage area that covers the South-East Asia and Australian areas of the world. The Chinese FY (Feng-Yang) satellite is parked at 104.degree. East and shows a substantially overlapping coverage area with the GMS-5 satellite. The European space agency's METEOSTAT-6 satellite, parked in a 0.degree. orbit, requires a license to decrypt and thus limits distribution for three days after observation. In contrast, the GOES, GMS and FY satellites have open reception and distribution via NASA-funded Internet links. Other satellites that perform similar operation include the Indian INSAT-1D, which is parked at 74.degree. East longitude, and the Russian system, GOMS/ELECTRO, which is not currently operational. A common feature of these different satellite systems is that they employ a spin scan or scanning visible imaging systems that require up to 20 minutes or longer to acquire a full disk image of the Earth. Furthermore, the systems use the long scan period to provide a variety of spatial resolutions, but all of which are more coarse than 1 km at the Nadir point.
There have been a number of proposals made in the past by various individuals and groups to place a camera on a large commercial communication satellite positioned in GEO. In each case, the camera would operate as a parasitic device, in that the camera would use the power and communication sub-system of the satellite to support its operational requirements. The most recent and most detailed examples, were made by Hughes Information Technology Corporation, a former subsidiary of Hughes Aircraft Company and the MITRE Corporation. These examples are discussed below.
The Hughes Proposal was described under various names such as "EarthCam", "StormCam", and "GEM" (Geostationary Earth Monitor) and involved a television style imaging system using a two dimensional charge coupled device (CCD) detector array to create an image of 756 pixels wide by 484 pixels high at intervals that range from between two minutes to eight minutes. The frame rate for this TV-style camera was determined by compression limitations in the satellite's meager 1-5 Kbps housekeeping data channel capacity. The Hughes Proposal described placing a digital camera on board one or more of Hughes' commercial telecommunication satellites (COMSAT). This parasitic camera was to operate using power provided by the COMSAT and deliver data to a Hughes ground operation center by way of a very low data rate housekeeping telemetry link. Data was then to be distributed to various users from this single command and control facility.
The system proposed employing cameras placed on board the Hughes satellites to be located at 71.degree. West, 101.degree. West, 30.degree. East and 305.degree. East longitude. Upon receipt, and after processing, data would be distributed via land line or communication satellite links to end-users. The single visible imaging system would operate with a zoom mode so as to achieve 1 km spatial resolution while building a composite hemispheric view from lower resolution images.
As presently recognized, the system proposed by Hughes was deficient in both its camera resources and communication systems infrastructure with regard to the following three attributes. The system proposed by Hughes did not provide real-time images (as defined herein) as a result of the delay between frames. Another deficiency was that real-time images cannot be distributed in real-time, due to the interval between frames and the slow data rate, as well as the single point data reception and distribution facility. Furthermore, the system proposed by Hughes was deficient in its inability to provide hemispheric (full disk images) in real-time. This limitation is due to the limited telemetry channel capacity, limited camera design and the time required to create a composite full disk image. Accordingly, as is presently recognized, the system proposed by Hughes neither appreciated the significance of providing an infrastructure that would be able to provide real-time images, distribute the real-time images, and provide for the compilation of a composite full disk images in real-time.
In 1995, the MITRE Corporation published a study that was performed in 1993. The study examined the use of parasitic instruments on commercial communications satellites for the dual purpose of augmenting government weather satellites and providing a mechanism for low cost test and development of advanced government environmental monitoring systems. The study performed by MITRE examined in some detail the application of newly developed megapixel, two-dimensional, CCD arrays to geostationary imaging systems. The study concluded that considerable gains in capacity could be achieved using the CCD arrays. Although the advent of CCD arrays as large as 4096.times.4096 were anticipated at the time the study was performed, the authors recognized that an array of 1024.times.1024 was the largest practical size available for application at that time.
Two distinct types of CCD array applications were considered, time-delay integration (TDI) and "step-stare" as alternatives to the traditional "spin-scan", or "flying-spot" imaging techniques. The TDI approach can be viewed as a modification of the "flying-spot" in that it uses an asymmetrical two-dimensional array, e.g., 128.times.1024, oriented with the long axis vertical so as to reduce the number of East-West scans. In this technique, every geographic scene element is sampled 128 times, which increases the signal-to-noise level. However, communication satellites are relatively unstable platforms. With a single pixel integration time on the order to milliseconds, spacecraft movement during the accumulation of over 100 samples may degrade the spatial resolution within any one scene element. This effect, which is in addition to the navigation and registration degradation due to scan line shift, is called "pixel spread". Image spread over long integration periods also degrades or precludes low illumination or night observing at visible wavelengths.
The "step-stare" approach was identified in the MITRE study as being the preferred technique. A large, two-dimensional CCD array in this technique is used to capture a portion of the image of the Earth. The optical pointing is incrementally "stepped" across the face of the Earth by an amount nearly equal to its field of regard at each step. The overlap ensures navigational continuity and registration correctness. With reasonable, but not extraordinary satellite stability, the frame time may be increased to milliseconds so as to achieve required levels of sensitivity without compromising navigational or registration criteria or image quality.
The MITRE study proposes the use of sub-megapixel arrays (1024.times.512). With a dwell time per frame of approximately 150 milliseconds, an entire composite full Earth disk image at 500 meter spatial resolution could be created from a mosaic of nearly 1,200 frames in relatively few minutes. The maximum exposure time to create an image in daylight is much shorter than 150 milliseconds for most CCD arrays. Furthermore, a reasonably stable satellite undergoes little motion during such a brief time interval thus reducing pixel spread. In order to ensure coverage of the entire Earth's surface, frames are overlapped by an amount defined by the satellite stability. This step-stare technique steps the frames in a line from North to South or from East to West, simultaneously exposing all pixels in an array. This ensures accurate registration and navigation of image pixels.
According to the MITRE study, the time between frames in a 500 meter resolution mosaic image of the Earth is three minutes (equal to the time needed to create the mosaic). As presently recognized, during this three minute interval, the motion of objects observed, such as clouds and smoke plumes, will cause the object's apparent shape to change in a discontinuous fashion. The continuity of successive observations will thus be compromised and degrade "seamless" coverage by an amount proportional to the velocities of the objects causing the shapes to apparently change. This degradation is called image smear and becomes more apparent as the time between frames increases image smear, thus putting a premium on decreasing the time to create a mosaic of the full disk image.
As presently recognized, with sufficient stability, it is possible for a CCD imaging system to allow the shutter to remain open to collect more light to enhance low illumination performance. The impact of CCD arrays in a step-stare scan on night imaging is not noted in the MITRE study. As recognized by the present inventor, low illumination imaging is possible by reducing the stepping rate, and allowing the camera field to dwell on the area of regard for a predetermined amount of time while integrating its emitted light. At the time of the MITRE study, time exposures to achieve night imaging capability would have increased the time to acquire a full disk image of the Earth to about 24 minutes, or about the same amount of time as the flying spot technique. Furthermore, the significance of obtaining real-time night images or the mechanisms needed to obtain the images was never appreciated, and thus not realized. In the MITRE study, data distribution was accomplished either by embedding a low data rate in the spacecraft telemetry, or directly to receive sites by preempting the use of one of the satellite's transponders. While the emphasis was on rapid full disk imaging, no special considerations were given to disseminate the data either live or globally.
In 1995, the Goddard Space Flight Center announced a study called the "GEO Synchronous Advanced Technology Environmental System" (GATES) that was expected to lead the development of a small satellite system equipped with a "push broom" scanning linear CCD array imaging device. This system was to use motion induced by the satellite's attitude control system to make successive scans of the visible Earth's disk. The satellite's attitude control momentum wheels would be used to slew the entire system back and forth 12 times while the field of regard of the camera's linear array is stepped from North to South to achieve a full disk scan in about 10 minutes. This system uses a 1,024 pixel long one-dimensional linear CCD array "flying spot" similar to, but much longer than, the GOES' eight pixel array.
As presently recognized, limitation with the GATES system is that live images are not possible, nor is night imaging. Data was distributed from a single receive site, via the Internet. A limitation with the Hughes proposed system, the MITRE system, and the GATES system, is that none of the systems appreciate the interrelationship between providing a real-time continuous monitoring capability of the entire Earth that is accessible from a geostationary Earth orbit, while providing high resolution images. In part, the limitation with all of the devices is that none of the devices would be able to reliably provide the "watchdog" high resolution imaging function that would provide a remote user with valuable real-time data of dynamic situations occurring at or near the Earth's surface.